Typically, orthogonal frequency division multiplexing (OFDM) systems suffer from peak to average power ratio (PAPR) problems. Due to cost and physical size limitations of user equipments (UE), examples of which are devices such as wireless cellular telephones or wireless enabled computers and PDAs, PAPR is a bigger problem in uplink (UL) transmission, in the direction from the UE to a base station, than downlink (DL) transmission, in the direction from the base station to the UE.
A solution that has been proposed for a current single antenna LTE standard is that the input sequence of an inverse discrete fourier transform (IDFT) is made such that the output PAPR is low. For example, for a physical uplink shared channel (PUCCH), data is discrete fourier transform (DFT) precoded before OFDM. Such a process is known as single carrier frequency division multiple access (SC-FDMA). For the physical uplink control channel (PUCCH) in a single antenna UE, a low PAPR orthogonal sequence (OS) may be used to spread the data across a given frequency band.
In some communication systems, such as for example, Long Term Evolution-Advanced (LTE-A) and Advanced Evolved UMTS Terrestrial Radio Access (A E-UTRA), UEs may be equipped with more than one antenna to enable higher peak data rates with more link reliability. In some cases the UE's multiple antennas may be used to provide open-loop transmit diversity (OLTD).
Some coding scheme candidates for using OLTD in the DL direction include: Space Time Block Coding (STBC); Space Frequency Block Coding (SFBC); Cyclic Delay Diversity (CDD); Pre-coding Vector Switching (PVS); and Radio Frequency Combining (RFC).
The various candidate schemes for DL each have their positive and negative qualities. STBC preserves the low PAPR property, but requires an even number of OFDM symbols per subframe. SFBC may increase the PAPR, but works for any number of OFDM symbols per subframe. CDD preserves the low PAPR property and works for any number of symbols per slot, but the performance is not as good as STBC and SFBC. PVS periodically uses some precoders using a known sequence for both the transmitter and receiver. RFC combines RF waves for data and reference symbols using a blind precoder at the receiver. The last two schemes suffer from poor performance in spatially correlated channels.
As discussed above, some of the candidate schemes for DL may not enable a low PAPR property. As such, the current schemes for DL may not be appropriate solutions for reliable UL transmissions.
When channel estimation is code division multiplex (CDM) based, such as the case for PUCCH, adding more antennas at the transmitter may also result in reconsideration of how channel estimation is to be performed.